code: def learn(self, obs, act, reward, next_obs, terminal):
      feed = {
          "obs": obs,
          "act": act,
          "reward": reward,
          "next_obs": next_obs,
          "terminal": terminal,
      }
      critic_cost = self.fluid_executor.run(
          self.learn_program, feed=feed, fetch_list=[self.critic_cost]
      )[0]
      self.alg.sync_target()
      return critic_cost
  
file_path: PARL/examples/tutorials/lesson5/ddpg/agent.py
imports: import numpy
  import parl
  from parl import layers
  from paddle import fluid
label_desc: Use image node_id 4 for calling the Agent obj's underlying member method code with example usage: obj.learn(obs, act, reward, next_obs, terminal) and returns: critic_cost